The subject invention relates generally to depolarization of light, and more particularly to an apparatus for producing broadband, unpolarized light.
The use of unpolarized light is of interest in the field of optics, especially in fiber-optic systems, because unpolarized light is relatively immune from randomly induced birefringences which can affect the state of polarization of light that is polarized. Such randomly induced birefringences are often caused by environmental perturbations such as temperature fluctuations or mechanical deformations. In fiber-optic gyroscopes, for example, depolarized and partially polarized light has been used to reduce sensor noise.
While the study of polarized light has drawn considerable attention ever since the time of Huygens, to date little work has been done on the development of unpolarized light sources or means for depolarizing light. Consequently, there is a paucity of information concerning light depolarization and unpolarized light sources having very low degrees of polarization.
Of the known techniques for depolarizing light the most effective are the devices of Lyot and Billings. The Lyot device, disclosed in Ann. Observatoire Astron. Phys. de Paris (Meudon), Tomi i Fasc. 1-28, 102(1928), consists of two quartz plates. These plates are X-cut and one is cut twice as thick as the other. The two plates are aligned parallel to each other such that their respective axes are at an angle of 45 degrees. The Lyot device is most efficient only when depolarizing light having a wide bandwidth. Furthermore, it is a rather bulky optical device because of the use of the retardation plates.
The Billings device is disclosed in Journal of the American Optical Soc., Vol. 41, No. 12, pp. 966-75 (1975). It consists of two variable wave plates of Z-cut, birefringent material. The plates are aligned such that their fast axes are at an angle of 45.degree. to each other. To vary the degree of retardation of the plates an electric field is applied to each plate. The intensity of the applied electric field is adjusted such that the retardation of one plate can be varied from 0 to 2.pi., and the retardation of the second plate, from 0 to 4.pi.. The Billings depolarizer, however, is an active device requiring a source of electric energy.
U.S. Pat. No. 3,433,553, Peters, discloses a light depolarizer which is passive and suitable for use with both narrow and broadband light. The Peters depolarizer consists of a retardation plate which has one of its surfaces roughened in order to provide a randomly varying thickness. This randomly varying thickness causes the production of a complete set of polarization states from linearly polarized light which is passed through the plate. This device has the disadvantage that the output beam has non-uniform spatial properties, i.e. each part of the beam has a different polarization state. Only when viewing the entire beam does the light appear to be depolarized.
In order to depolarize light which is other than linearly polarized, Peters combines a plurality of the roughened plates, each disposed with its optical axis at an angle to the optical axis of an adjacent plate. It can be readily seen, however, that the degree of depolarization is dependent on the number of plates utilized. Thus, in order to obtain complete depolarization many plates would have to be used resulting in a bulky optical apparatus.